The present invention relates to a method for planarizing a deposited film by chemical mechanical polishing for use in production of a semiconductor device. More particularly, it relates to a method for forming a buried interconnect in a multi-layer interconnect process or forming an isolation region in an isolation process.
A method for forming a buried interconnect by the chemical mechanical polishing (CMP) will now be described as a conventional example with reference to FIGS. 11A through 11C and 12A through 12C.
First, as shown in FIG. 11A, an interconnect groove 3 is formed by photolithography and dry etching in an interlayer insulating film 2 of silicon dioxide deposited on a semiconductor substrate 1. Thereafter, as shown in FIG. 11B, a barrier metal layer 4 of, for example, a tantalum nitride film is formed over the interlayer insulating film 2 including the inside faces of the interconnect groove 3. The barrier metal layer 4 is thus formed in a small thickness over the interlayer insulating film 2 because copper used for forming a copper interconnect can easily diffuse into the silicon dioxide film of the interlayer insulating film 2 so as to degrade the insulating property of the interlayer insulating film 2.
Next, as shown in FIG. 1C, a seed layer 5 of copper is formed on the barrier metal layer 4 by sputtering, and then, the seed layer 5 is grown into a copper film 6 by electroplating as shown in FIG. 12A. In this case, the copper film 6 is grown to have a thickness larger than the depth of the interconnect groove 3 so that the interconnect groove 3 can be completely filled with the copper film 6. Thus, an initial level difference 7 is formed in the copper film 6 above the interconnect groove 3.
Then, as shown in FIG. 12B, an excessive portion of the copper film 6 present outside the interconnect groove 3 is removed by the CMP, thereby forming a buried interconnect 6A from the copper film 6. Finally, a portion of the barrier metal layer 4 present above the interlayer insulating film 2 is removed by the CMP as shown in FIG. 12C.
Since tantalum nitride used for forming the barrier metal layer 4 for preventing diffusion of copper is a very stable material, it is difficult to simultaneously remove the copper film 6 and the barrier metal layer 4 by the CMP.
Accordingly, in order to form the buried copper interconnect 6A, the copper film 6 and the barrier metal layer 4 should be separately polished. Specifically, at a first stage of the CMP, the copper film 6 alone is removed by the polishing and the polishing is stopped at the surface of the barrier metal layer 4. A slurry used at the first stage of the CMP preferably has a polishing rate for tantalum nitride sufficiently higher than that for copper. Then, at a second stage of the CMP, the portion of the barrier metal layer 4 present above the interlayer insulating film 2 is removed by using a slurry suitable for polishing tantalum nitride. The slurry used at the second stage of the CMP preferably has a polishing rate for copper equivalent to or lower than that for tantalum nitride. Through the CMP thus carried out in the two stages, the buried interconnect 6A can be formed without eliminating the copper film 6.
FIG. 13A shows an ideal cross-sectional shape of the buried copper interconnect 6A, and FIG. 13B shows an actual cross-sectional shape of the buried copper interconnect 6A. Specifically, the copper film 6 is polished during the CMP until the top face of the buried copper interconnect 6A is placed at a level lower than that of the interlayer insulating film 2. Accordingly, a plane face as shown in FIG. 13A cannot be obtained but unevenness designated as dishing is caused on the buried interconnect 6A as shown in FIG. 13B.
When the dishing is caused on the buried interconnect 6A, a variety of problems may occur as follows: Since the height of the buried interconnect 6A is lowered, the interconnect resistance may be increased. In forming a multi-layer interconnect structure, polishing residue of a copper film or a tantalum nitride film may be caused in a buried interconnect in an upper layer, which can cause short-circuit of the interconnect or can increase focal shift in the photolithography so as to cause a pattern defect.
Accordingly, in order to form a high performance buried interconnect, it is very significant to reduce dishing caused on the buried interconnect 6A.
Alternatively, in forming an isolation region by filling an isolation groove with an insulating film in the isolation process, dishing may be caused on the isolation region. In this case, the isolation region is reduced in its thickness, and hence, a leakage defect may be caused between devices to be isolated or a pattern defect may be caused.
Accordingly, in order to form a high performance isolation region, it is very significant to reduce dishing caused on the isolation region.
There are some known causes for increase of dishing, against which countermeasures have been considered.
For example, the dishing tends to increase as the width of an interconnect is increased. This is because of elastic deformation of a polishing pad, and the upper limit in the width of an interconnect is provided at the stage of circuit design as the countermeasure.
Also, the dishing tends to increase as the polishing pad is softer. This is also because of the elastic deformation of the polishing pad, and a hard polishing pad is used as the countermeasure.
Furthermore, the dishing tends to increase as over-polishing is increased. The over-polishing is performed at the ultimate stage of the polarization process in order to completely remove an excessive portion of the copper film partly remaining on the substrate surface. The over-polishing is effective means for preventing short-circuit of an interconnect derived from polishing residue of the copper film and hence is indispensable, but excessive over-polishing may increase the dishing so as to increase the interconnect resistance and cause polishing residue in a buried interconnect in an upper layer. Therefore, the over-polishing should be sufficiently carefully performed. In other words, the over-polishing should be performed to a necessary and minimum extent. Excessive over-polishing is performed because of in-plane variation in the thickness of the deposited copper film and in-plane variation in the polishing rate in the CMP. Therefore, when such in-plane variations are reduced, the over-polishing can be avoided so as to reduce the dishing.
Another cause of increase of the dishing is the thickness of the copper film. Specifically, when the copper film has a too small thickness, an interconnect pattern is exposed before completely eliminating the initial level difference, and hence, the remaining initial level difference directly leads to the dishing of the interconnect. On the other hand, when the copper film has a too large thickness, the in-plane variation in the thickness of the copper film and the in-plane variation in the polishing rate in the CMP are both caused, and hence, the over-polishing is increased so as to increase the dishing.
In consideration of the aforementioned problems, an object of the invention is reducing dishing caused in completing over-polishing in the CMP.
In order to achieve the object, the method for planarizing a deposited film of this invention comprises the steps of forming a groove in a surface portion of a substrate; forming a deposited film on the substrate so as to fill the groove; eliminating an initial level difference formed in the deposited film due to the groove by subjecting the deposited film to a first stage of chemical mechanical polishing with a relatively high rotation speed and a relatively low pressure; and removing a portion of the deposited film present outside the groove after eliminating the initial level difference by subjecting the deposited film to a second stage of the chemical mechanical polishing with a relatively low rotation speed and a relatively high pressure.
In the method for planarizing a deposited film of the invention, since the first stage of the chemical mechanical polishing is performed with a relatively high rotation speed and a relatively low pressure, the deposited film can be planarized in a short polishing time, so as to improve the planeness of the deposited film attained in completing the first stage of the chemical mechanical polishing. Also, since the second stage of the chemical mechanical polishing is performed with a relatively low rotation speed and a relatively high pressure, the portion of the deposited film present outside the groove can be removed in a short polishing time, so as to suppress dishing.
In the method for planarizing a deposited film, the deposited film preferably has a thickness 1.6 through 2.0 times as large as a depth of the groove.
Thus, surface unevenness caused in completing the first stage of the chemical mechanical polishing can be suppressed to 20 nm or less, and the time required for the second stage of the chemical mechanical polishing can be shortened. As a result, the dishing can be suppressed.
In the method for planarizing a deposited film, the first stage of the chemical mechanical polishing is preferably performed until a thickness of the deposited film remaining on the substrate becomes larger than zero and not larger than 50% of a depth of the groove.
Thus, the time required for the second stage of the chemical mechanical polishing can be shortened, so as to reduce in-plane variation in the thickness of the deposited film caused during the second stage of the chemical mechanical polishing. As a result, the dishing can be further reduced.
In the method for planarizing a deposited film, the first stage of the chemical mechanical polishing is preferably performed until a thickness of the deposited film remaining on the substrate becomes larger than zero and not larger than 200 nm.
Thus, the time required for the second stage of the chemical mechanical polishing can be shortened, so as to reduce the in-plane variation in the thickness of the deposited film caused during the second stage of the chemical mechanical polishing. As a result, the dishing can be further reduced.
In the method for planarizing a deposited film, in-plane variation in a thickness of the deposited film attained when the first stage of the chemical mechanical polishing is completed is preferably 5% or less.
Thus, the dishing can be further reduced.
In the method for planarizing a deposited film, surface unevenness remaining on the substrate when the first stage of the chemical mechanical polishing is completed is preferably larger than 0 and not larger than 20 nm.
Thus, the dishing can be further reduced.
The method for planarizing a deposited film of this invention preferably further comprises a step of performing conditioning of a polishing pad between the first stage of the chemical mechanical polishing and the second stage of the chemical mechanical polishing or at the beginning of the second stage of the chemical mechanical polishing.
Thus, the performance of a slurry used at the second stage of the chemical mechanical polishing to hold abrasive grains can be improved so as to improve in-plane uniformity in the polishing rate. As a result, the dishing can be further reduced.
In the method for planarizing a deposited film, the groove preferably has a width of 1 xcexcm through 100 xcexcm.
Thus, the effect of the method for planarizing a deposited film of this invention can be definitely exhibited.
Preferably in the method for planarizing a deposited film, the groove is an interconnect groove and the deposited film is a conducting film, and the step of removing a portion of the deposited film present outside the groove through the second stage of the chemical mechanical polishing includes a sub-step of forming a buried interconnect from the conducting film.
Thus, a buried interconnect with less dishing can be definitely formed.
In this case, the method for planarizing a deposited film preferably further comprises a step of forming a barrier metal layer between the interconnect groove and the conducting film, and it is preferred that the conducting film is a copper alloy film and that the barrier metal layer is a tantalum nitride film.
Preferably in the method for planarizing a deposited film, the groove is an isolation groove and the deposited film is an insulating film, and the step of removing a portion of the deposited film present outside the groove through the second stage of the chemical mechanical polishing includes a sub-step of forming an isolation region from the insulating film.
Thus, an isolation region with less dishing can be definitely formed.
In this case, the method for planarizing a deposited film preferably further comprises a step of forming an inversion preventing layer on a bottom of the isolation groove, and the insulating film is preferably a silicon dioxide film.